Add WasmEntryPoint directive to change/specify entrypoints

[dpfens]

Adds the WasmEntryPoint directive described in #25. The entrypoint still defaults to _start

1. WasmEntryPoint directive sets a new entrypoint field to WasmConfig
2. WasmConfig assigns the entrypoint field to the WasmExecutionCtx
3. The specified entrypoint is invoked by WasmExecutionCtx::run.

[gzurl]

Thanks @dpfens for this contribution! LGTM with minor comments.

Can we add a unit test for this new directive?

[dpfens]

Can we add a unit test for this new directive?

@gzurl I added a WasmEntryPoint usage to mod_wasm/httpd.conf to verify it works in the demo/Docker container. Is there an existing way to create a unit test for a directive which would be executed during the GitHub Action? I'm not seeing it.

Hi @dpfens, it's OK to doing this way for now. We don't have yet in place a way to test the C code.

[gzurl]

Added son minor comments. Thanks!!